The first, and obvious, point to make about dimensioning braze fillets on drawings is that once a braze-fillet size is specified on a drawing, it must be inspected and measured to verify compliance. This can be a meaningless and very expensive waste of time for a manufacturer when it comes to braze fillets.
The American Welding Society (AWS) document that is most commonly used for specifying how to properly place such requirements on a drawing is AWS A2.4 "Standard Symbols for Welding, Brazing and Nondestructive Examination." That document includes numerous illustrations about how to use so-called "welding-arrow symbols" to specify fillet sizes on drawings. Be very careful.
To someone that is somewhat new to the world of brazing, it might be assumed after reviewing the information in A2.4 that it is proper (and even desirable) to dimension the braze fillets on drawings. This is NOT the intent of the writers of A2.4, and persons who try to control braze fillets by specifying their dimensions on drawings are doing something that is VERY UNWISE.
Remember, if you specify a dimension, then it must be verified by inspection. What would then happen if a braze fillet turned out to be a bit smaller than the allowed dimensional range specified on a drawing? If properly handled, it would need to be sent back to the brazing department for additional brazing filler metal (BFM) to be applied. What if the added BFM now caused the dimension of the braze fillet after re-brazing to be just a little bit larger than the maximum allowed by the tolerance spread shown on the drawing? The excess would have to be ground away to bring the dimension into allowed tolerance bands shown on the drawing. That's a lot of extra work and wasted time to achieve something that isn't even important. All this work, and the many hours consumed to do it, can be very real consequences of not understanding the information given in AWS A2.4 regarding braze-fillet dimensioning.
When I spoke to one of the key people at AWS, who put this information into the document some years ago, that person told me that the braze-fillet dimensioning info shown in A2.4 was NOT designed to get people to start dimensioning braze fillets. Instead, it was put there so that IF someone wanted to know how to properly dimension a braze fillet, the info contained in A2.4 could guide that person into the correct methods for such dimensioning. Again, it must be stressed – IF someone needed to dimension a braze fillet.
Some readers may already be objecting to some of my statements thus far. You may say, "I have to control the size of the fillet because I can't allow the BFM to be in such-and-such a place on the part." So, fine, merely state on the drawing the following: "No BFM is allowed in such-and-such a location." Don't ever specify a braze-fillet size as a way to achieve this. Merely specify which areas on the parts need to be free of any BFM flow.
I cannot think of any REAL need to ever dimension braze fillets. Remember that braze fillets are the natural outcome of the brazing process, and they merely give evidence that the joint has been brazed. The braze-fillet (meniscus) is NOT a controllable part of the brazing process, as far as being able to supply a specific-size fillet is concerned. All the goodness of a brazed joint is found inside the brazed joint, between the faying surfaces. It is the filling of the capillary space between the closely fitted faying surfaces that will provide joint strength, leak tightness (hermeticity) and fatigue resistance. The outside of the joint has little, if anything, to do with joint integrity.
Never use braze fillets to "help spread the stress" at the edge of the joint. That is a VERY poor design criterion to use in a brazed-joint design. "Stress spreading" is strictly a function of the design of the base-metal components, which must be designed in such a way that stresses do not concentrate at the corner/edge of a brazed joint. You NEVER intentionally build up a braze fillet to help "spread the stress" at the edge of a joint. That's "weld think," and although it might work for a weld fillet, it will NOT be effective for braze joints and can actually be harmful to a brazed joint.
A braze fillet should ideally be very small and show a concave shape at the edge. Large braze fillets are castings with inherent weakness.
Therefore, rather than dimension a braze fillet, a good braze call out would be: "There shall be evidence of brazing filler metal at all edges of a brazed joint." That's all. No dimensions; just a statement that the braze fillet (meniscus) shall be visible. Keep the fillet small. Remember, all the good things about a braze joint are happening INSIDE the joint, not in the outside fillets.
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